Scientists unveil new technique to sort nanotubes by length

Washington, May 17 : Researchers at the National Institute of Standards and Technology (NIST) have come up with a technique to sort carbon nanotubes by length.

The novel technique involves the use of high-speed centrifuges, they say.

The researchers say that many potential applications for the so-called single wall carbon nanotubes (SWCNTs) depend on the lengths of such microscopic cylinders.

According to them, one of the most important features of their new technique is that it should be easily scalable to produce industrial quantities of high-quality nanotubes.

The researchers have revealed that their latest study builds on a previous work that showed in 2006 that nanotubes could be separated by "chirality" (a measure of the twist in the carbon atom sheet) by spinning them in a dense fluid in an ultracentrifuge tube, due to a relationship between chirality and buoyancy.

Their latest study showed that a variation of the same technique could separate nanotubes by length.

The research team observed that while the nanotubes ultimately would move to a point of equilibrium in the centrifuge tube dictated by their buoyancy, they would move at different rates depending on their lengths due to friction.

"When we spin the centrifuge, it turns out that the longer ones move faster. We basically just run a race and the longer ones move farther in the same amount of time. Eventually they get separated enough in position that we can just pull off layers and get different lengths," says researcher Jeffrey Fagan.

Although several other techniques have also been shown to sort nanotubes by length, this is the first approach that may be scaled up to produce commercially important quantities of nanotubes in a given length range.

The researchers also claim that their method removes much of unwanted junk, particularly metal particles, from the batch of carbon nanotubes.